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The best thing you can do is study up on it. Know what kind of heat exchanger is in the furnace as soon as you walk up on it. (serpentine, tubular, drum....etc...) Knowing what kind of heat exchanger you are dealing with will tell you where to start your search...and what tools will be needed to do that search. Visual is always the best, if you can find a way to see the crack...so can your homeowner.

I had this in my favorites from long ago don't know how true all of it is but read it and make up you own mind.

If you smoke a couple of cigarettes, or someone had been smoking a cigar in the last hour, you could hit 11ppm CO in your home. Cigarette smokers exhale between 4 and 9 ppm CO all the time. And, here's the funny part, if you're in a city, it's possible that the background CO rises to that level during the day. The mechanic's meter might hit 12-15 ppm standing in the middle of your back yard!

Cracked heat exchangers do NOT produce CO. Period. This is a myth that's been handed down from the days of gas conversion burners, and old furnaces with "pot belly" heat exchangers.
Solid fuels, such as wood, always produce carbon monoxide when they are burned. Gas and liquid fuels may produce no CO or very little.

I would call around and find an outfit that knows how to use their test equipment. I'll bet the mechanic came in the house, switched on his CO tester, and then began looking for CO. If that's the case, he doesn't have a clue! The calibration kit for a decent CO meter costs MORE than a cheap CO meter. The calibration kit should be used every time the CO meter is deployed. It's the only way to guarantee that the CO meter is responding to CO and that it's calibrated to give the correct reading.

You can take any CO meter on the market and make it read what ever you want at low levels.

Besides, if you had been cooking, using the oven, heating water on the stove, or had a door or window open that could cause your flue to down draft, you might see 5-20ppm in the house for a period of time. An oven will produce anywhere from 20 to 600 ppm when operating. Most run in the 30-100ppm range.

(Before I get a bunch of "hate mail", let me remind everyone that the ANSI limit for CO output from a residential oven is 800ppm.)

Without knowing how your home, the furnace and the flue are situated, it's hard to make a guesstimate about where the CO is coming from. But I'll bet big bucks, it's NOT from your furnace.

Besides, 11-12ppm CO may be ambient conditions and nothing to really worry about. It could also be "left over" output from cooking or some other activity. To make it even more complicated, the cheap CO meters and many residential CO detectors are cross-sensitive to other gasses, like methane, butane, alcohol, aerosols, etc. I've seen "Pam" cooking spray put a CO detector into alarm.

Explain your situation to the service manager for each prospective company and ask the following questions.
1. Do you use CO meters that are calibrated every year?
2. Are the calibration stickers visible on the meters?
3. Do you use a calibration kit to set the meters before testing?
4. Do you have equipment to check the draft in my flue?
5. Will you "red tag" and disable my furnace if a physical crack in the heat exchanger is found? (You don't want that outfit checking your furnace in the dead of winter!)

If you get a "NO" answer to any of the first four questions, move on to the next service company.

Don't know who wrote this "probably some ticked off HO" and I am not saying I agree with them but just thought I would lay it out there.

__________________________________________________ _______________________
“Experience is a hard teacher because she gives the test first, the lesson afterwards". -Vernon Law-

Mr Bill....thats quite a read! I'm guessing pissed off home owner as well. Love the fact that nothing is even mentioned about the AGA....their the governing body that says any and all heat exchangers found with a crack cannot be used until replaced. Cracks cannot be fixed and may not be leaking CO into the home....however the potential for this poison to enter your home is there and that is why we shut them down. I just wish homeowners would ask themselves....if I have a bomb in my basement....should I just ignore it or maybe have it removed???